1. l Field of the Invention
This invention relates to ink jet printing and particularly to a method and apparatus for controlling the velocity of ink drops in an ink jet printer.
2. Description of the Prior Art
In ink jet printers of one well-known type, drops of a field-controllable ink are formed and propelled from a nozzle toward a print medium. Ink is supplied to the nozzle under pressure sufficient to cause the ink to issue from the nozzle as a continuous stream. Drop forming means such as a piezoelectric or magnetostrictive transducer attached to the nozzle or other means such as an electromagnetic excitor in the vicinity of the stream generates perturbations in the stream to cause it to break into individual drops of substantially uniform size and spacing. Field control devices located in the vicinity of the trajectory of the stream are regulated in accordance with data signals to cause the individual drops to be dispersed onto the print medium to form data patterns. To insure proper placement of the drops it is important that the velocity of the drops while moving along the trajectory be maintained as constant as possible.
The need for maintaining the velocity of the ink drops substantially constant to insure good print quality is well recognized in the art. One velocity correction scheme is described in U.S. Pat. No. 3,600,955, issued on Aug. 24, 1971, to V. E. Bischoff. This velocity correction scheme is based upon determining the phase difference between electrical pulses generated by a drop detector located adjacent to the stream and the electric pulses applied to the drop charging tunnel. A resultant time variable pulse representing drop velocity is used to operate a meter calibrated to display the degree and direction of any velocity error. A human operator while observing the meter operates the ink pump to change the pressure to make the desired adjustment in drop velocity.
In a publication by W. T. Pimbley in the IBM Technical Disclosure Bulletin, on page 948+ of Volume 16, No. 3, August 1973, velocity correction is achieved by determining phase variances between drop generating pulses applied to an ink stream excitor and drop sensing pulses of a drop detector located a fixed distance apart in the direction of the ink stream trajectory.
In the prior art schemes the maximum detector pulse phase shift, i.e. the maximum velocity error for which an accurate velocity correction can be made is 180.degree.. Another way of considering this is that a drop will be directly aligned with the detector for ideal velocity when the drop generator or the drop charging tunnel is pulsed. When a velocity change occurs at the same drop generating frequency, the drop will not be aligned with the detector. Thus, if there is a decrease of velocity, the drop that was previously aligned with the detector will not have travelled as far and will be located upstream for the detector when the drop generator is pulsed. Similarly, an increase in velocity will cause the drop to be located downstream of the detector when the drop generator is pulses. When proper velocity correction is made, the drop located closest to the detector will align with the detector. Thus, for example, the fast stream will be slowed and the drop near the detector will shift upstream and align with the detector at the time when the generator is pulsed. Accurate velocity correction according to prior art schemes can only be made when the distance between the drop associated with the nth wavelength and the detector is less than one-half of a drop wavelength at the time when the drop generator or drop charger is pulsed. The prior art velocity correction schemes are not effective to correct for gross velocity errors, that is, an error in which the shift is more than one-half a wavelength at the nth drop location relative to the detector. In other words, where a gross velocity error exists, the number of drops between the drop generator and the drop detector may be incorrect. An adjustment using the prior art schemes may not correct for the number of drops that should be present in the stream. Thus, the prior art velocity correction schemes might actually show no velocity error when, in fact, the number of drops in the drop stream at the time the velocity error correction is made may actually be too few or too many.